dataNode是数据存储的最小单元，保存节点数据内容，ACL控制列表，节点状态，父节点引用和子节点列表，同时也提供了子节点列表更新的方法
DataNode
属性
//节点签名，通过路径，数据，状态计算的long型数据
// the digest value of this node, calculated from path, data and stat
private volatile long digest;
// indicate if the digest of this node is up to date or not, used to
// optimize the performance.
volatile boolean digestCached;
//字节数组存储数据
/** the data for this datanode */
byte[] data;
/**acl long标识
* the acl map long for this datanode. the datatree has the map
*/
Long acl;
/**datanode持久化的状态表示
* the stat for this node that is persisted to disk.
*/
public StatPersisted stat;
/**子节点列表
* the list of children for this node. note that the list of children string
* does not contain the parent path -- just the last part of the path. This
* should be synchronized on except deserializing (for speed up issues).
*/
private Set<String> children = null;
/**
* 空set集合Collections.emptySet()
*/
private static final Set<String> EMPTY_SET = Collections.emptySet();
类关系图
StatPersisted类
属性
private long czxid;//节点创建时zxid
private long mzxid;//节点更新的zxid
private long ctime;//节点创建时间
private long mtime;//节点更新时间
private int version;//节点更新次数
private int cversion;//子节点的更新次数
private int aversion;//节点acl更新次数
private long ephemeralOwner;
private long pzxid;//子节点被最后更新的zxid
思考：
什么是zxid?
zxid包含两部分，the epoch and counter
64位数字
高32位用于epoch
低32位用于counter
递增唯一全局id